The present invention relates to hardboard panels, and more specifically to hardboard panels having improved smooth, dense surfaces and methods for making these.
Many different methods have been developed for making hardboard products by reducing wood or other lignocellulose products to fine particles, forming these particles into a unitary mat, and consolidating the mat into a solid board by applying heat and pressure. Two known methods for making hardboard are the wet process and the dry process.
In the wet process a pulp mat is prepared from aqueous slurry of wood fibers and formed into a thick sheet in a manner similar to the conventional paper manufacturing process, and the mat is pressed into a fiber board panel.
The term "dry process" or "dry formed" usually indicates that the fibers are conveyed in a gaseous stream rather than a liquid stream to a felter and formed into a mat which is consolidated into hardboard by the application of heat and pressure. If desired, the particles or chip board materials may be mechanically conveyed in the dry process for felting or forming. The main distinguishing feature of the dry process is the absence of a liquid transport medium. Even though the process is termed "dry", the fibers can have a moisture content of up to 100% of the weight of the dry fibers. Moisture in the range of 5 to 8% is generally required for good consolidation.
It has been found that in forming thick panels by the dry process, particularly those above 1/4 inch in thickness, it is difficult to secure hard, smooth surfaces. Soft surfaces which have low abrasion resistance, and are excessively adsorptive of paint or adhesive material, are often obtained. It is believed that the migration of moisture away from the surface before the resin is heated sufficiently to flow, inhibits proper consolidation of the fibers at the surface of the mat and results in soft surfaces. It has been found that panels are improved by the addition of moisture to mat surfaces prior to their entering the press. To prevent the rapid migration of the moisture away from the surface of the panel before the resin flows to bond the fibers, the use of surface added moisturizers for hardboard manufacture was suggested in now abandoned U.S. Ser. No. 289,370, filed Aug. 31, 1972 as identified in U.S. Pat. No. 4,053,339. In that application, a mixture of water and ethylene glycol was sprayed onto the mat surface prior to pressing and resulted in a superior surface. However, this process had the undesirable side effect of an increased tendency of the panel to stick to the platens or configured molding caul. It has been the experience with this type of process that a subsequent sanding procedure may be required after molding. This causes increased costs for those panels which can be improved by sanding, but is wholly unacceptable for panels that are to be finished without sanding.
In U.S. Pat. No. 4,053,339, Story and Gibson proposed solving this problem by applying a dry paper overlay, such as newsprint, on the hardboard panel prior to final curing. It is disclosed that this procedure resulted in a good bond between the paper overlay and the panel by the use of a mixture of phenolic resin and the preferred plasticizer, ethylene glycol. While overlay panels made according to that procedure did have superior finishing properties, good paint holdout, high resistance to scuffing, and could be easily and cleanly cut, they had the disadvantage that the extra operation of applying the dry paper overlay introduces an additional degree of complexity into the process.
While not directed to the immediate problem of providing a smooth, dense surface layer in a hardboard panel, U.S. Pat. No. 2,629,674 to Ericks discloses the treatment by impregnating preformed mats with a mixture of a glycol and a thermosetting resin to improve dimensional stability of the board. The patentee does not disclose surface treatment of a preformed mat prior to pressing with caul plates to provide improved surface characteristics of the final board, but teaches adding his compositions to the entire stock prior to formation to obtain improved dimensional stability.
Other workers, such as Heritage and Roberts in U.S. Pat. No. 2,872,337, have suggested adding surface treating agents to impart a glossy, hard surface to pressed board products. The patentees specifically suggested thermosetting resins such as phenol-formaldehyde resins. Alternatively, they suggest thermoplastic treating agents such as vinyl resins which can be employed in solutions, emulsions or suspensions using water, alcohol or other liquid carrying medium. While this reference offers a concept of broad scope, it does not provide much specific guidance for employing specific operable treatments based on the thermoplastic resins which provide improved surfaces and good release from the mold.
There remains a need to develop a simple and effective method for preparing hardboard panels with smooth, dense surfaces, and there obviously also remains a need for a more economically produced panel having these desired surface characteristics.